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Hydraulic Determinations When the topographic shape of the stream channel and adjacent lands In the valley have been adequately represented by cross-sections. and when the flow rates and volumes have been determined, the next step IS a hydraul. ic analysis. This analysis will descnbe the depth of flow along a selected study reach and present it In profile view. These depth'of.flow determinations assist in establishing regulatory flood elevations for the 10.. 50.. 100.. and 500'year floods and allow the preparation of a floodplain map to show the floodplain in plan view. The 10-,50-. 100. and 500-year water surface elevations and profiles must be calculated by using the Corps of Engineers' HEC'II Computer Program or an acceptable backwater' step procedure. A part of the hydraulic analysIs IS the selection of roughness coefficients. "n's," for each cross-section based on the eXisting conditions along the stream and floodplain. These "n's," whiCh represent the roughness or resis' tance to water flow created by vegetation, rocks. and other channel features. are essential for appropriately descnblng the flow. Past fiood data. if avail. able, are used to verify the adopted roughness coefficients. taking into con. sideration any alteration in the channel subsequent to those floods. If there are stream gauging stations within the study reach. the computed water sur. face profiles are checked against the rating curves for those stations to venfy in the n-value. Another part of hydraulic analysis is determining the effects of culverts and bridges. They can affect flood flows by constncting them or obstructing them completely. All cuiverts and bridges are evaluated for the following hydraulic characteristics: . propensity for reduced conveyance capacity due to debris blockage of the culvert 0' b"dge: . damming effect due to undersized structures; and . diversion of flood flows or washout of structures due to the above charactef/stics. The potential for blockage of structures such as bridges and culverts and the subsequent reduction In conveyance IS based on watershed charactens. tics such as erodibility of channel banks. amount and type of vegetation along the stream, and size and character of the waterway. . . Blockage may be artificially accounted for in the hydraulic calculations by simulating: . an increase in the width of bridge piers; . a raising of the streambed elevation; or . a reduction In the waterway opening by a percentage, Once all of the hydraulic van abies have been considered and a backwater analysis has been completed. floodwater surface elevations are portrayed In the floodplain report in two ways. Fltst. flood profiles graphically ShOw the relationship, In profile or side VIew, of the water surface elevations to the channel bottom at a particular pOint along the stream. Second. a reference table shows elevations at each floodplain cross' section used In the study. The flood elevations presented in the profIles and the reference table should be calculated to a precIsion of :to.5 foot for the 10" 50., 100.. and 500'year flood events and referenced to 1929 Mean Sea Leve/lMSLI datum. The flood elevations and outlines tor an actual flood event may lIary from these figures. It is difficult to evaluate all the vanables which will affect hydraulic calculations. Variables include scounng of the channel due to high velOCities. sedimentation, variations in channel characteristics between cross-sections. amounts of debris accumulation, and "mltatlons in field surveys and mapping techniques. Despite all of these englneenng limitations. a detailed hydrologic and hydrauliC investigation IS stili the most reliable method for determining flood elevations. .